348 Dr. J. R. Ashworth on Magnetic Hysteresis 



the field-coefficient (« H ), which are closely connected with 

 the critical field, differ from what are observed by a factor 

 of the order 10 6 . 



(2) In seeking to resolve this apparent discord between 

 theory and experiment, the following considerations must be 

 kept in view. 



The intensity of magnetization, unlike the density of a 

 fluid, is a vector quantity. Thus zero intensity does not 

 necessarily imply the absence of an intrinsic field as zero 

 density would imply the absence of an intrinsic pressure. 



With ferromagnetics below the critical temperature zero 

 intensity means that the orientation of the molecular magnets 

 is such that the resultant field for points outside the magnet 

 vanishes ; the intermolecular field or intrinsic field is still 

 existing and its magnitude may be not very different from 

 what it is under a high intensity of magnetization. 



If magnetization is regarded as the orientation into more 

 or less complete alignment of molecular magnets moving in 

 an intrinsic field, the action of an externally applied field 

 must be mainly a directive one. For example, it is a familiar 

 fact that an external field of a few units leads to very hi<^h 

 inductions, and, on the other hand, very high inductions 

 (of as much as 16,000 lines of force per sq. cm.) are easily 

 annulled by a reversed field of a few units. It is evident 

 in the latter case that the external field does not counter- 

 balance the induction, but only controls it directively. 



The external field, when applied to an unmagnetized ferro- 

 magnetic, may be imagined as acting immediately on a few 

 of the molecular magnets, which are more easily affected than 

 the majority, and these act on neighbouring molecules which 

 in their turn re-act on others, and so the effect of the initial 

 stimulus rapidly spreads and an intensity of magnetization is 

 produced which puts in evidence the intrinsic field. Thus a 

 small external field does not create but may control a large 

 intrinsic field *. 



It was from this point of view that the statement was made 

 that a latent heat of magnetization, if it exists, must be very 

 small, since in magnetism the rapid acquirement of magnetic 

 intensity only involves the orientation but not the creation of a 

 magnetic intrinsic field, whereas in fluids the rapid conden- 

 sation of a saturated vapour to liquid involves the creation 



* Tlie remarks here made apply equally well to the Kinetic theory. 

 The intrinsic field given by Weiss as NI has a magnitude of the order 

 10 T gausses, but it may be controlled by a small external field, and physically 

 this small held is not to be neglected. The retention of the external field 

 in the treatment of the Kinetic theory in Pdrt II. is thus justifiable. 



